BEIRUT - Efforts to secure the release of Lebanese soldiers and policemen held captive by the Nusra Front have been obstructed by last minute demands from the Syrian al-Qaida-linked group, the head of a Lebanese security agency was quoted as saying on Monday.Major General Abbas Ibrahim, head of General Security, told al Joumhouria newspaper the government had fulfilled all its commitments to secure their release. The men were expected to be released over the weekend in a deal that would include the release of a number of Islamist prisoners jailed in Lebanon.The security personnel were taken captive in August 2014 during an incursion from by fighters from the Nusra Front and Islamic State into the border town of Arsal. The authorities have been trying to negotiate the release with Qatari mediation."We showed our flexibility on many of the demands by Nusra Front, and completed all the commitments to implement the deal," Ibrahim said. "It got to a point where we could not comply with paralyzing demands that the kidnappers made at the last minute."Four of those held have been killed. Nusra is still holding 16 security personnel, according to a security source. Nine soldiers believed to be held by Islamic State are not included in the negotiations.Local media began circulating reports on Friday that a deal was imminent, but by Monday morning none had been struck.Prime Minister Tammam Salam cancelled a trip to Paris for a global climate change summit in order to follow up on the continuing negotiations.Violence from the Syrian war has repeatedly spilled over to Lebanon, with bombings in Beirut, fighting in the northern city of Tripoli, and rocket attacks on Bekaa Valley towns close to the frontier.